A common configuration for small electric furnaces used for melting glass is a generally basin or bowl-shaped open-topped container lined with refractory bricks and having a central outlet orifice at the bottom through which molten glass may flow by gravity. The outlet is generally defined by an outlet structure having a vertical orifice in which a metering needle having a variable diameter is adjustably positioned to control the flow rate of the molten glass. Examples of such prior furnace constructions are published in U.S. Pat. Nos. 3,912,488 and 3,983,309 and in the prior art patents therein cited and discussed.
The extremely high temperatures involved in glass melting and the tendency of flowing molten glass to erode the surface of the confining chamber combine to create very substantial design and durability problems, particularly in the area of the furnace outlet. Temperatures within the furnace interior may be in the range of 2500 to 3000 degrees F. That environment requires that the outlet passageway or orifice for the molten glass be formed of a refractory metal capable of resisting the combined effect of the high temperatures and the erosion or deterioration which normally results from the flow of molten glass over its exterior surface and through its central orifice. The aforementioned patents recommend using molybdenum, tungsten, columbium, tantalum and alloys of such metals for the orifice block. Such materials, however, readily oxidize if exposed to ordinary atmosphere at temperatures in excess of about 700 degrees F. (in the case of molybdenum). The relatively great temperature differential which would therefore be required between the closely spaced inner and outer ends of the orifice block has heretofore precluded the use of a single block of material for this purpose.
Prior attempts to solve this problem, such as disclosed in the above-referenced patents, include the use of a two-stage orifice block comprising upper and lower portions of dissimilar materials spaced apart by an insulator, so that the lower portion, which is exposed to the atmosphere, can be kept at a lower temperature. The two-stage concept also permits selection of optimum materials having the required physical properties for the contrasting environments at the inner and outer ends of the outlet structure. Another solution involves flooding the area surrounding the exterior of the orifice block with an inert protective atmosphere, to isolate its heated exposed surface from contact with oxygen. Cooling jackets containing circulating water have also been used in conjunction with various orifice block configurations.
These earlier attempted solutions have not been entirely satisfactory. The use of multiple-stage orifice blocks wherein the lower or outer block is fabricated of platinum has been suggested in the prior art. However, platinum is an extraordinarily expensive material. Similarly, the use of protective atmospheres surrounding the outer end of the orifice block also introduces cost and space penalties and additional equipment to be maintained.
Accordingly, it is the principal object of the present invention to provide an improved orifice block for an electric glass-melting furnace which provides assured durability to withstand high temperatures, erosion and oxidation tendencies, while requiring minimal use of expensive materials and supplementary construction and equipment.
In summary, the present invention involves the use of an orifice block, preferably formed of a single piece, extending from the melting zone all the way through the wall and lining of the furnace, with its outer end substantially flush with a cooling jacket containing circulating water. The block is preferably formed of molybdenum, and the portions of the block at and adjacent to its outer end have a minimal external diameter to reduce the surface area exposed to the atmosphere and to reduce the mass of block at the outlet end which must be cooled. A shoulder in the exterior contour of the block forms a transition from the reduced diameter portion to the larger diameter of the main portion of the block. This shoulder, along with the periphery of the reduced diameter portion, is directly exposed to the cooling jacket, to provide maximum cooling for the outer end of the block. The combination of water cooling, minimal mass and the very minimal surface area exposed to the atmosphere eliminate any danger of oxidation at the exterior surface, and obviate the need for any protective atmosphere.